This invention is related to waste-water treatment, and more particularly to the disposal of paper mill sludge off-gases and sewage sludge off-gases. It is most useful in the disposal by incineration of (i) the sludge, (ii) off-gases generated incident to the treatment of the sludge, and (iii) off-gases generated from trickling filters and the like, so that heat produced in the incinerator may be used to generate steam. This process is equally applicable to `non-autogenous` sewage sludge, so referred to because it does not generate enough heat upon burning to sustain combustion without the addition of heat from an external source ("auxiliary fuel"), and to `autogenous` sewage sludge.
As those skilled in the art will appreciate, the energy balance around a sewage disposal facility is of critical significance and determines the economics of its operation. Failure to recover energy generated within an incinerator for the sludge in a non-autogenous system must be paid for by supplying external fuel. Thus, a critical economic appraisal of the energy balance becomes keyed (a) to maintaining the temperature of a gas as high as possible if it is to be used for combustion, so as not to heat cold gas in the combustion zone, irrespective of the calorific (or BTU) value of the gas, and (b) to utilizing high calorific value gases for combustion rather than low calorific value gases, because the size of the incinerator defines its volumetric flow rate limitation.
`Wet-scrubbing` is as basic a "unit operation" in the chemical engineering art as `heat exchange`, `filtration`, `distallation`, `extraction`, and several others. The first step in the design of a processing plant is to consider which one or more of the several unit operations would best serve the overall purpose of the plant to be built; the second step is to consider how and where each chosen unit operation is to be used; and, the third step is to consider the details of designing the equipment to perform each chosen operation, consistent with the best engineering practices. As will be readily recognized by those skilled in the art, once a choice of unit operations is made, and specifications are provided as to what each operation must accomplish, and how, a competent design falls into place, albeit with deceptive ease. The cost is usually more than one expects it to be. This invention is directed to the unobvious choice of plural unit operations in a sewage sludge wast disposal plant, and the unique combination of steps incident to their implementation, which steps result in a process for solving a particular and peculiar problem in a suprising effective manner.
Referring again to `wet-scrubbing`, it will be recognized that this is a commonplace unit operation in the treatment of `stack gas`, namely, effluent gases from a sewage sludge incinerator. Wet-scrubbers are conventionally used to decrease the amount of gas-borne pollutants discharged into the atmosphere. In a wastewater treatment plant, water for wet-scrubbing may be effluent water which has been treated in the plant (`treated effluent`). The water may also contain a small amount of caustic soda (NaOH), sufficient to neutralize or otherwise react with components of the off-gases, usually less than about 5 percent by weight (% by wt) of the water. In some wet-scrubbers, the water may contain a small amount of potassium permanganate (KMnO.sub.4), usually less than about 5% by wt of the water. In still other wet-scrubbers, the water may contain powdered activated carbon, in amount less than about 1% by wt, or activated sludge, preferably in an amount in the range from about 50 parts per million (ppm) to about 5000 ppm, or mixtures of the foregoing. By `wet-scrubbing` I refer, in this specification, to all the foregoing wet-scrubbing applications, and `water` for wet-scrubbing refers to water which may contain either NaOH or KMnO.sub.4 in solution, or powdered activated carbon, or activated sludge, or any combination of the foregoing.
Numerous engineeringly sophisticated wet-scrubbing devices have been used to "clean up" stack gas economically. The purpose and effect of all such devices is to remove a major portion of the fly ash, as much other particulate matter as they can, and such water-soluble compounds from the stack gas as are readily soluble. Necessarily incidental to this purpose, because cool water is generally the liquid phase in the wet-scrubbing device, the temperature of the stack gas is lowered to a temperature in the range from about 100.degree. F. to about 200.degree. F. while it becomes saturated, that is, has 100% relative humidity. Since providing cool water adversely affects the economics of the plant, the temperature of the stack gas is maintained no lower than is necessary to discharge a visually acceptable plume.
Despite such wet-scrubbing, the concentration of particulate matter in the stack gas is still relatively high, and the relatively low temperature of the stack gas deleteriously affects the operation of all equipment in contact with it. To combat the problem, U.S. Pat. No. 3,382,649 (the '649 patent) teaches increasing the temperature of the wet-scrubbed stack gas, and various schemes are provided to do so. Heating of the wet stack gas, as disclosed in the '649 patent, is unrelated to the overall demands of the incinerator because of the type of sludge it is burning, the sole purpose being to minimize the corrosive effect of stack gas and negate its proclivity to encrust the equipment with solid matter.
However, recognizing that it is desirable to provide preheated air to the combustion zone, the '649 patent also teaches cooling the rotatable central shaft and connected rabbling arms of a multiple hearth furnace (also referred to herein as "incinerator" or simply "furnace"), with combustion air. As is well-known in the art, and described in U.S. Pat. Nos. 2,181,190, 2,286,309, 2,317,941, and 4,215,637 inter alia, the teachings of which, as to the construction of multiple hearth furnaces, are incorporated by reference thereto, the central shaft and interconnected rabbling arms are in open flow communication.
Shaft-cooling gas ("CG") to cool the shaft and rabbling arms is provided by a blower which blows cool air upwards through the shaft. This cool air is heated in the shaft, thus keeping the shaft relatively cool in the hot furnace, and the air leaves the top of the shaft as shaft-heated air at a temperature in the range from about 250.degree. F. to about 600.degree. F., to be recycled as combustion air into the multiple hearth furnace. The flow of cool air required to keep the shaft cool during operation is referred to as the "normal cooling air capacity" of the shaft. A minimum air velocity through the center shaft must be maintained in order to assure satisfactory heat transfer, while maximum air velocity is determined by the pressure drop through the shaft. Therefore flow of gas through the shaft has definite limits.
Off-gases, other than the stack gas, generated in a "wet-oxidation" process, have also been wet-scrubbed for several purposes, including (i) decreasing their temperature so as to provide a visually acceptable plume, (ii) decreasing the concentration of water-soluble ingredients thought to be responsible for the malodorous characteristic of sludge treatment off-gases (hereafter simply "odorous" off-gases to connote their characteristically unpleasant smell), and (iii) decreasing the concentration of ingredients thought to be at least in part responsible for the corrosivity of the off-gases.
When sludge treatment includes air injection to achieve some oxidation, the process is called "wet oxidation", and is described in greater detail in U.S. Pat. Nos. 3,697,417 and 3,359,200, to Teletzke et al and Gitchel et al respectively, the disclosures of which as to sludge treatment are incorporated by reference herein as if fully set forth. Such off-gases include those separated and discharged from the reactor, or a separator vessel, or the decant tank of a waste-water treatment plant, and are collectively referred to herein as "sewage sludge off-gas". Any such off-gas discharged, and particularly decant tank off-gas ("DTO") is typically warm (100.degree.-200.degree. F.), and usually water-saturated and odorous.
The decant tank is so termed because cooled sludge is allowed to thicken and settle in the tank and supernatant liquid is decanted. This decant tank is also referred to as a `thickening tank` (as in Teletzke et al, supra) or an `oxidized sludge tank` (as in Gitchel et al). As disclosed in the '417 patent, DTO is now wet-scrubbed and discharged to the atmosphere.
For example, is a particular sewage plant built in Hollywood, Florida, decant tank off-gas ("DTO") which had been diluted with fresh air, was wet-scrubbed with cooling water and discharged to the atmosphere. The odor was unacceptable. When wet-scrubbed, its temperature dropped less than 10.degree. F., but sufficiently to provide a better plume. However, the CO.sub.2, CO, CH.sub.4 and total hydrocarbon content ("THC") were reduced insignificantly, and there was no improvement in the odor of the discharge.
In another sewage plant built in Minneapolis-St. Paul, MN, DTO was wet-scrubbed with water which cooled the DTO from about 120.degree. F. to about 70.degree. F. Again, the odor was unacceptable. This cooled wet-scrubbed gas was not burned because it was deemed uneconomical to do so.
Since it was known that combustion of off-gases improved their odor, off-gases have been burned directly in the furnace without wet-scrubbing, primarily for odor control, for example at a plant built at Grand Haven, MI, and described in a brochure entitled "Thermal Sludge Conditioning Report: Grand Haven, Michigan" and distributed by that city. This form of odor control was effected in accordance with the suggestion of a paper entitled "Odor Control for Thermal Sludge Conditioning Units" delivered by William M. Copa in 1974.
In another article, titled "Odor Control: an operator's guide" by Henry, J. G. and Gehr, R., Journal WPCF, Vol 52, No. 10, pgs 2523-2537, at pg 2533, October 1980, "direct combustion" is suggested for high odor concentration and small volume of combustible gases with at least 4,000 KJ/m.sup.3 (131 BTU/ft.sup.3) heating value, not typical of a sludge off-gas. For comparison, it is noted that the heating value of natural gas is about 1000 BTU/ft.sup.3. No suggestion is made that the odorous gas be wet-scrubbed before it is burned, for any reason, in either reference.
Direct combustion has also been used, after the DTO has been wet scrubbed to saturate it, in an autogenous furnace to avoid its overheating. In an autogenous furnace burning off-gases, predominantly DTO was directly burned in the combustion zone without wet-scrubbing, for example, at the Kalamazoo, MI Wastewater Treatement plant. In another autogenous furnace built in Cedar Rapids, IA., odorous off-gases, predominantly DTO, were burned after wet-scrubbing because the moisture in the off-gases helped prevent overheating of the furnace, though an additional benefit was the decrease of corrosivity of the off-gases due to scrubbing.
In no instance was a low heating value off-gas, that is, having a heating value less than about 100 BTU/ft.sup.3, first wet/scrubbed to a temperature below about 90.degree. F., and then burned in an incinerator with the specific purpose of recovering such little heating value as the off-gas may have. As is evident from the foregoing, there was no reason to do so.